Fats and oils constitute an important component of human diet. They are a source of essential fatty acids such as linoleic, linolenic and arachidonic acids, and act as vehicles for vitamins as well as being a source of calories. Fats and oils are also widely used to enhance the texture and palatability of foods. Their varied uses necessitate a wide range of melting and crystallization properties.
The physical properties of a fat or oil are determined by (i) the chain length of the fatty acyl chains, (ii) the amount and type (cis or trans) of unsaturation present in the fatty acyl chains, and (iii) the distribution of the different fatty acyl chains among the triacylglycerols that make up the fat or oil. Those fats with a high proportion of saturated fatty acids are typically solids at room temperature while triacylglycerols in which unsaturated fatty acyl chains predominate tend to be liquid. Thus, hydrogenation of a triacylglycerol stock ("TAGS") tends to reduce the degree of unsaturation and increase the solid fat content and can be used to convert a liquid oil into a semisolid or solid fat. Hydrogenation, if incomplete, also tends to result in the isomerization of some of the double bonds in the fatty acyl chains from a cis to a trans configuration. Concerns over potential health implications of excessive consumption of fatty acids with trans double bonds (e.g., via margarines, shortenings or frying oils), has led to interest in the manufacture of low- or zero-trans spreadable fats.
By altering the distribution of fatty acyl chains in the triacylglycerol moieties of a fat, randomization can produce changes in the melting, crystallization and fluidity characteristics of a triacylglycerol stock. As well as leaving the overall degree of unsaturation (e.g., as measured by the Iodine Value) of a triacylglycerol stock unchanged, interesterification reactions typically do not generate additional trans double bonds.
With rising concerns over potential dietary effects of trans unsaturated fatty acids, interesterification provides an important alternative to partial hydrogenation in the production of plastic fats such as shortenings and margarines. Interesterification has been used in the oil industry for about a century, although the mechanism is still not well understood by the industry. Currently, interesterification is conducted with experimentally determined catalyst dosages and reaction times to reach a thermodynamic equilibrium (i.e., complete randomization) of the distribution of fatty acyl chains in a triacylglycerol stock. In practice, interesterifications are run with an excess of catalyst to ensure completion interesterification and the randomized product is then characterized by measurement of its physical properties, such as melting point and solid fat content. Herein the terms "randomization," "complete interesterification" and "complete randomization" are used interchangeably.
Interesterification has been demonstrated as a method to prepare plastic fats and modify fat crystals. Stock oils and fats commonly available as raw materials for interesterification have varying qualities and levels of purity. Because of this, the experimental dosage of catalyst required for initiating or completing interesterification can vary widely. In some cases, because of inactivation reactions, the catalyst dosage may be insufficient to initiate or complete interesterification, resulting in production delays and cost increases. To overcome this limitation, interesterification reactions are typically run using a substantial excess of catalyst. The physical properties and/or composition of the reaction product must be assayed upon completion of the interesterification. While this approach can ensure initiation and completion of the reaction, it does not permit the degree of control of the reaction rate necessary to reproducibly achieve a specific level of partial esterification.